This invention relates to an electrophotographic plate.
In electrophotographic materials using photoconductive substances as a photosensitive material, there have mainly been used inorganic photoconductive substances such as selenium, zinc oxide, titanium oxide, cadmium oxide, etc. But some of them have strong toxicity, which results in causing a problem of disposal.
On the other hand, photosensitive materials using organic photoconductive compounds are generally weak in toxicity and advantageous in transparency, flexibility, lightweight properties, surface smoothness, price, and the like compared with the inorganic photoconductive substances. Thus, the organic photoconductive materials have widely been studied recently. Among them, complex type electrophotographic plates having separate charge generating function and charge transport function, that is, electrophotographic plates comprising a charge generating layer and a charge transport layer laminated on an electroconductive substrate, have been developed rapidly, since sensitivity can be remarkably improved compared with known electrophotographic plates using organic photoconductive compounds.
When a photosensitive layer is directly formed on an electroconductive substrate, particularly when the photosensitive layer comprises a charge generating layer and a charge transport layer and the charge generating layer is directly contact with the electroconductive substrate, cissings and unevenness are produced due to surface defects such as slight relief, scratches, stains, deposits and the like formed on the electroconductive substrate surface, which results in easily making the film thickness of the charge generating layer non-uniform. Thus, when such a complex type electrophotographic plate is applied to an electrophotographic device using, for example, the Carlson method and printed, there often arises a problem in that such surface defects appear on images formed as they are, for example, to produce black stains (small black points having a diameter of 200 .mu.m or less) on the whole white background or white stains (white unprinted points having a diameter of 200 .mu.m or less) on the whole black background, which results in lowering the image quality.
In order to solve such a problem, it is proposed to form a resin layer between the charge generating layer and the electroconductive substrate (that is, on the surface of the electroconductive substrate) in order to cover defects, stains, deposits, etc., on the surface of the electroconductive substrate and to make the charge generating layer uniform in thickness. Such a resin layer is generally called as an undercoating layer or an intermediate layer.
As the undercoating layer or intermediate layer, there are proposed to use layers make from resins such as polyamide resins, polyurethane resins, polyvinyl butyral resins, melamine resins, casein, phenol resins, epoxy resins, ethylene-vinyl acetate copolymer resins, ethylene-acryliric acid copolymer resins, etc., or to use resin layers containing electroconductive substances such as electroconductive metallic powders and metal complexes (e.g. Japanese Patent Unexamined Publication Nos. 61-110153, 61-163346, 61-20049, 61-204640, 61-204641, 61-240247, 61-254951, 61-258258, etc.).
Even if the undercoating layer is formed between the charge generating layer and the electro-conductive substrate, printing defects such as black stains and white stains are reduced to some extent at room temperature, but not removed completely. When the printing circumstances are at 30.degree. C. or higher, particularly at 35.degree. C. or higher, the black stains and white stains are increased to remarkably lower printing quality. This still retains a problem in that there can only be obtained unstable electrophotographic plates having a large change in printing quality.